The weather conditions at the time of the occurrence were ideal for the flight training exercises and were not a factor in the accident. Flight instructors are aware of the dangers of allowing a spiral to develop at low altitude and especially of continuing below 2 000 feet agl. It could not be determined why a spiral was continued to an altitude from which a safe recovery could not be performed. The wing impact damage indicates that the aircraft was probably entering a recovery attitude prior to striking the trees. It was not determined whether the failure of the left seat back occurred in-flight or as a result of impact forces. Control of the aircraft would likely have been maintained by the instructor seated in the right seat had the left seat back failed in-flight. The severity of the impact damage and fragmentation of the airframe made it impossible to determine if a flight control malfunction had occurred; however, the impact evidence indicates that the aircraft was probably entering a recovery attitude prior to impact. The sudden absence of engine noise that captured the witness's attention likely resulted from the pilot initiating the spiral recovery procedure. It was evident that the engine was capable of producing power. The following Engineering Branch report was completed: LP40/99--Airspeed Indicator ExaminationAnalysis The weather conditions at the time of the occurrence were ideal for the flight training exercises and were not a factor in the accident. Flight instructors are aware of the dangers of allowing a spiral to develop at low altitude and especially of continuing below 2 000 feet agl. It could not be determined why a spiral was continued to an altitude from which a safe recovery could not be performed. The wing impact damage indicates that the aircraft was probably entering a recovery attitude prior to striking the trees. It was not determined whether the failure of the left seat back occurred in-flight or as a result of impact forces. Control of the aircraft would likely have been maintained by the instructor seated in the right seat had the left seat back failed in-flight. The severity of the impact damage and fragmentation of the airframe made it impossible to determine if a flight control malfunction had occurred; however, the impact evidence indicates that the aircraft was probably entering a recovery attitude prior to impact. The sudden absence of engine noise that captured the witness's attention likely resulted from the pilot initiating the spiral recovery procedure. It was evident that the engine was capable of producing power. The following Engineering Branch report was completed: LP40/99--Airspeed Indicator Examination For undetermined reasons, the aircraft entered a spiral manoeuvre that continued below the CARs minimum aerobatic recovery altitude. The aircraft was probably entering a recovery attitude at the time of impact with the trees.Findings as to Causes and Contributing Factors For undetermined reasons, the aircraft entered a spiral manoeuvre that continued below the CARs minimum aerobatic recovery altitude. The aircraft was probably entering a recovery attitude at the time of impact with the trees. The instructor pilot was certified and qualified in accordance with existing regulations to conduct the flight training lesson. The company maintenance records indicate that the aircraft was certified, equipped, and maintained for flight in accordance with existing regulations. There was no evidence of airframe failure or engine malfunction prior to or during the flight. The aircraft engine was capable of producing power at the time of the accident. Weather was not a factor in the accident.Other Findings The instructor pilot was certified and qualified in accordance with existing regulations to conduct the flight training lesson. The company maintenance records indicate that the aircraft was certified, equipped, and maintained for flight in accordance with existing regulations. There was no evidence of airframe failure or engine malfunction prior to or during the flight. The aircraft engine was capable of producing power at the time of the accident. Weather was not a factor in the accident.